Fix callback races in glibmm when source is destructed
Fix callback races in glibmm when source is destructed

It is normal situation when glib main loop iterates sources on one
thread where it checks whether source is still active and its callback
functions can be called and glibmm Source being destroyed on other
thread. Glibmm should check once again that callback_data and
callback_funcs fields are still valid and GSource was not marked
as inactive while its callback handlers are called.

Fixes #41

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm